We propose to study the structure and function of the leukocyte hemeprotein, myeloperoxidase. Two tryptic peptides specifically involved at the active site will be isolated. The first of these will be a heme-bearing fragment of the heavy subunit; the second, the cysteine containing portion of the small subunit. The free, buried cysteine residue of this subunit appears to be associated with the heme group, possibly as the distal ligand. At the same time, physical studies, including nuclear magnetic resonance, electron microscopy, X-ray crystallography, infra-red spectroscopy and EPR spectrometry will be applied to the protein structure and the heme environment. The heme will be released from the protein and studied by NMR and spectral techniques. In the area of mechanism and function, the chemiluminescent reaction of chloride peroxidation will be examined to determine whether singlet oxygen is formed. Also, rapid reaction studies will be applied to the chloride peroxidase cycle to determine the roles of compounds I and II.